Two ISREs functioning cooperatively regulate ISG expression in West Nile virus infected IFNAR-/- MEFs

We previously identified a subset of interferon stimulated genes (ISGs), including Irf7, Irf1, Oas1a and Oas1b, upregulated by a West Nile virus (WNV) infection in wildtype mouse embryo fibroblasts (MEFs) after viral proteins had inhibited type 1 interferon (IFN)-mediated JAK-STAT signaling and also in WNV-infected STAT1-/-, STAT2-/-, IFNAR-/-, IRF3-/-, IRF7-/-, and IRF3/7-/- MEFs. In this study, ISG upregulation by WNV infection was inhibited in RIG-I/MDA5-/- but not in single knockout MEFs. ISGs upregulation was detected in WNV-infected IRF1-/- and IRF5-/- but was minimal in IRF3/5/7-/- MEFs, suggesting redundant IRF involvement. ISG upregulation by WNV infection in IFNAR-/- MEFs was confirmed by RNA-seq. We previously showed that a single proximal interferon stimulated response element (ISRE) in the Oas1a and Oas1b promoters bound the ISGF3 complex during IFN-dependent upregulation. In this study, we used wild-type and mutant promoter luciferase reporter constructs to identify critical regions in the Oas1b and Ifit1 promoters for IFN-independent transcriptional regulation. Two ISREs were required for both promoters. Mutation of these ISREs in an Ifit1 promoter DNA probe reduced in vitro complex formation. An NF-?B inhibitor decreased Ifit1 promoter activity in cells and in vitro complex formation. IRF3 and p50 binding was detected by ChIP for four upregulated ISGs with two proximal ISREs in their promoters. The data indicate that IFN-independent ISG expression of some ISGs is upregulated by two ISREs functioning cooperatively and the binding of a complex consisting of IRF3, 5, and/or 7 and an NF-?B component(s) as well as other as yet unknown factors. Overall design: Analysis of differentially expressed genes in mock-infected and West Nile W956-infected IFNAR-/- MEFs at 12 and 24 hours post infection